ENH: add Pstream chunked transfer detail

- to service both List and Map exchanges with limited message sizes
  (termed 'data chunking' here) add a PstreamDetail for walking and
  dispatching.  Like other Detail components, the API is subject
  to (possibly breaking) changes in the future at any time.

  The regular exchangeBuf detail has this type of signature:

  PstreamDetail::exchangeBuf
  (
      const UList<std::pair<int, stdFoam::span<const Type>>>& sends,
      const UList<std::pair<int, stdFoam::span<Type>>>& recvs,
      ...
  )

    Where [rank, span] is the tuple pack.

  The basic idea is to pre-process the send/receive buffers and
  marshall them into a flat list of [rank, span] tuples.

  The originating buffers could be any type of container (List or Map)
  which is then marshalled into this given sequence that can be
  processed in source-agnostic fashion.

  If data chunking is required (when UPstream::maxCommsSize > 0)
  it is possible to make a cheap copy of the rank/address information
  and then walk different slices or views.

ENH: replace private static methods with PstreamDetail functions

- simpler to update locally.